1. Field of the Invention
The present invention relates to a magnetic bearing device stably carrying a rotary shaft, and particularly relates to a magnetic bearing device, which can stably and rotatably carry a rotary shaft even when variations occur in load applied to a rotary shaft, a crossflow fan device for excimer laser provided with the magnetic bearing device, a program for executing a computer to execute feedback control on a magnetic bearing, and a computer-readable record medium storing the program for executing the computer to execute feedback control on the magnetic bearing.
2. Description of the Background Art
A magnetic bearing is used for a spindle, which is employed in a spindle motor for a machine tool or a turbo-molecular pump for rotation at a high speed of tens of thousands of revolutions per minute. In such a high-speed region, a problem of unstable rotation may occur due to a resonance mode or a gyro effect of the rotary shaft. For suppressing such problems, gains and control parameters of a control device controlling the magnetic bearing have been adjusted.
In connection with a spindle for a machine tool, compensation has been made for changes in specific frequency, which may occur due to change of an end tool.
Further, such a technique has proposed that uses a coil current in a magnetic bearing for detecting a load applied to a rotary shaft, and such a technique has also been proposed that a load applied to a rotary shaft, a positional deviation signal of the rotary shaft and a rotation speed thereof are determined, and a gain of a control device controlling the magnetic bearing is controlled based on the load, positional deviation signal and rotation speed thus determined (Japanese Patent Laying-Open Nos. H1-137302, H7-24602, S60-16149 and H11-93953).
However, a conventional magnetic bearing bears a motor torque of several Newton-meters at the most, and the load exerts only a slight influence on stable control of the magnetic bearing. Therefore, such a technique has not substantially been proposed that can stably control the magnetic bearing in view of the influence exerted on the magnetic bearing by the load.
As a technique related to compensation for large load variations of excimer laser, Japanese Patent Laying-Open No. 2002-089489 has disclosed a technique; in which the number of magnetic bearings receiving a load is increased (i.e., magnetic bearings, which are redundant with respect to a degree of freedom of a rotary shaft, are arranged), and control parameters of the magnetic bearings are changed in accordance with a rotation speed of a fan attached to the rotary shaft. Further, in connection with a digital control system of a magnetic bearing using a DSP (Digital Signal Processor), there has been proposed a technique, in which a memory has prestored control parameters corresponding to a rotation speed, and are changed in accordance with the rotation speed. Further, there has been proposed a technique, in which a sensor determines a motor drive current, and a rigidity of a magnetic bearing is changed in accordance with the motor drive current thus sensed. In this technique, a reference value of the motor current is predetermined, and a gain in control of the magnetic bearing is increased when the motor current exceeds the reference value.
However, the fan attached to the rotary shaft exerts a large load, and a motor driving the rotary shaft produce a large torque of ten-odd Newton-meters or more so that a large load is applied to the magnetic bearing. Further, the magnetic bearing undergoes large load variations between a state of receiving substantially no load, e.g., in a vacuum condition and a state of receiving the maximum load. This results in a problem that the magnetic bearing cannot be stably controlled in all the states.
Even if the stable control can be achieved, minute adjustment or regulation is required, and/or a cycle time must be reduced for avoiding rapid acceleration and deceleration so that other performances are sacrificed for these and other reasons.